20140215 08:40:52 HOW TO PROVE IT, PART 4 proof by personal communication: 'Eightdimensional colored cycle stripping is NPcomplete [Karp, personal communication].' proof by reduction to the wrong problem: 'To see that infinitedimensional colored cycle stripping is decidable, we reduce it to the halting problem.' proof by reference to inaccessible literature: The author cites a simple corollary of a theorem to be found in a privately circulated memoir of the Slovenian Philological Society, 1883. proof by importance: A large body of useful consequences all follow from the proposition in question.